Clinical Pearls & Morning Reports
Published January 23, 2019
Similar to West Nile virus, Powassan virus is a flavivirus, but it is transmitted by the Ixodes scapularis tick and maintained in a rodent reservoir. Until recent years, this viral infection was very rare in New England, with no cases reported to the CDC in most years. However, 5 cases in northern New England and 13 cases in Massachusetts were reported from 2013 to 2016. Read the latest Case Records of the Massachusetts General Hospital here.
Q: What is the most commonly identified cause of viral encephalitis in the United States?
As a group, viruses are the most commonly identified cause of acute encephalitis and meningoencephalitis. Herpes simplex virus is the most commonly identified cause of viral encephalitis in the United States. Because early treatment with intravenous acyclovir lowers morbidity and mortality, prompt consideration of herpes simplex virus is of critical importance.
Q: How specific are the clinical features of Powassan virus encephalitis?
The clinical spectrum associated with Powassan virus encephalitis is similar to that associated with eastern equine encephalitis virus infection and West Nile virus infection. Results of routine laboratory studies are typically normal, and the CSF white-cell count is usually less than 1000 per cubic millimeter. Parkinsonism and involvement of the basal ganglia and thalamus are common.
A: Anaplasmosis, which is due to Anaplasma phagocytophilum, is common in New England and is transmitted by Ixodes scapularis, the black-legged deer tick. Patients with anaplasmosis typically present with an acute febrile illness, often with a headache. However, encephalitis is rare in patients with anaplasmosis. Lyme disease, which is due to Borrelia burgdorferi, is a well-known cause of aseptic meningitis and of cranial and radicular neuritis, manifestations that occur in the early disseminated stage of infection. However, encephalitis is rare in patients with neuroborreliosis. Infection with B. miyamotoi, a distant cousin of B. burgdorferi, is an emerging cause of human disease in the northeastern United States. The most common clinical presentation resembles that of anaplasmosis, including similar laboratory abnormalities. An association between B. miyamotoi infection and meningoencephalitis has been described, albeit in immunocompromised persons
A: Powassan virus can be challenging to diagnose, because direct detection of the virus in blood or CSF with PCR assays or culture techniques is typically possible only during the prodromal phase of illness. During this phase, patients may not seek clinical attention, and if they do, there may be no reason to suspect Powassan virus infection. Once the illness enters the encephalitic phase, viremia is often undetectable and the diagnosis usually relies on antibody testing. In the first days or weeks of encephalitis, it is usually possible to detect only an IgM antibody response. This presents a challenge, because first-line tests for Powassan virus IgM antibodies, such as enzyme-linked immunosorbent assays or similar immunoassays, are more likely to produce false positive results than tests for Powassan virus IgG antibodies. The false positives are mainly caused by cross-reactivity with antibodies directed against other flaviviruses. A positive serum or CSF test for Powassan virus IgM antibodies should be confirmed with a test for neutralizing antibodies, such as the plaque reduction neutralization test or a similar assay.